To do Knife Edge loops, and most other 3-D type stunts you need great vertical pull, not top end speed. You also need Prop wash over the Rudder, not airspeed. Get an APC 17x6 and be amazed at the improvement. My YS 120 FZ with an APC 17x6 at 9300 rpm develops over 17 lbs thrust (accurately measured).
Also, a Rudder requires much more force to be deflected in a 70 MPH air stream than it does in a 40 MPH prop wash. Slow that plane down and you can throw the tail around much better.